1. Field of the Invention
The invention relates generally to navigation systems and more specifically to global positioning system (GPS) devices with rapid time-to-first-fix and re-acquisition performance.
2. Description of the Prior Art
Global positioning system receivers use signals received from typically three or more overhead satellites to determine navigational data such as position and velocity, and such systems may also provide altitude and time. GPS signals are available worldwide at no cost and can be used to determine the location of a vehicle, such as an automobile, to within one city block, or better. Dual-frequency carrier GPS receivers typically track a pair of radio carriers, L1 and L2, associated with the GPS satellites to generate accumulated delta-range measurements (ADR) from P-code modulation on those carrier frequencies and at the same time track L1 C/A-code to generate code phase measurements. Carrier frequency L1 is allocated to 1575.42 MHz and carrier frequency L2 is positioned at 1227.78 MHz. Less expensive receivers tune only one carrier frequency, and therefore do not have adequate information to compute the local ionospheric delays that will appear as position errors. At such frequencies, radio carrier signals travel by line-of-sight.
Thus buildings, mountains and the horizon can block reception.
The constellation of GPS satellites in orbit about the earth presently comprises approximately seventeen individual satellites. Each transmits one of thirty-two unique identifying codes in a code multiple access arrangement. This allows all of the many GPS satellites to transmit in spread spectrum mode at the same frequency (plus or minus a Doppler frequency shift of that frequency as results from the satellite's relative velocity). Particular satellites are sorted out of a resulting jumble of signals and noise by correlating a 1023 "chip" code to one of the thirty-two pseudo random number (PRN) sequence codes that are preassigned to individual GPS satellites. These codes are not necessarily being transmitted in phase with one another. Therefore, "finding" a GPS satellite initially involves searching various carrier frequencies, to account for Doppler frequency shift and oscillator inaccuracies, and searching for a code match, using 1023 different code phases and twenty or more possible correlation code templates.
In large cities with many tall buildings, one or more of the GPS satellites being tracked by a particular receiver, may be temporarily blocked. In some situations, such blockage can prevent all the overhead GPS satellites from being tracked and such outages can last for several minutes. GPS signals also become unavailable to vehicles moving through underground or underwater tunnels.
At least one background art GPS five-channel receiver directs all of its channels to focus on one satellite at initial turn-on, as is indicated by the user display on such receivers. This addresses the problem of satellite signal frequency uncertainty that exists due to Doppler effects and local oscillator inaccuracies in the receiver. A search for a particular satellite in the apparent Doppler frequency spectrum is conducted in parallel by segmenting the possible Doppler frequency spectrum into as many segments as there are receiver channels and appointing each of the several receiver channels to attend to a search within a respective segment. The single largest uncertainty stems from the random frequency possible from typical local oscillators at start-up. Therefore, the apparent Doppler frequency will be totally unknown, regardless of whether the actual Doppler frequency is known, as might be available if the present position is known.